In the manufacture of a commonly used communications cable, a core comprising a plurality of electrical insulated conductors is enclosed with an aluminum shield or with aluminum and steel shields. The aluminum shield is designed to protect the cable core from lightning damage and from electrical disturbances when installed in the field, while the steel shield provides mechanical and rodent protection for the cable core. When two shields are used, the steel shield typically is the outer of the two. The shields are formed from continuous tapes which are often corrugated and which are wrapped longitudinally about the cable core with corrugations of the longitudinal edge portions of the shield intermeshed to form an overlapped seam.
Early on, it was common practice to join the overlapping edge portions of the seam by soldering, or by an adhesive to form a hermetic seal to prevent moisture penetration of the cable core. With the advent of a waterproofing system for communications cable, of the type shown, for example, in U.S. Pat. No. 3,607,487, issued to M.C. Biskeborn et al on Sept. 21, 1971, it was no longer necessary for the shield seam to be joined to form the hermetic seal as long as longitudinal edge portions of the metallic shield were overlapped and a closed seam produced by suitable forming or working of the metal tape.
In manufacturing such a cable with an unjoined seam, it was found that an outer overlapping edge portion of the metallic shield tended to rebound subsequent to forming and to project outwardly, rather than to have its corrugations in a nested mating relation with the corrugations of the adjacent inner edge portion. The shield and the core tended to form a partially completed cable with a distorted non-circular periphery in which the outer edge of the shield possibly protruded into or through an outer plastic jacket that was extruded around the shield. These deficiencies manifest themselves in ruptured cable most frequently during the reeling or pay-off of the cable upon or from a reel, particularly if done in a relatively cold environment.
Another problem evolves because in order to conserve plastic materials and to provide a more uniform jacket, the shielded cable core is desirably pressure extruded, rather than tubed, which requires that the shielded cable core have a substantially circular configuration. A shield which presents a substantially circular configuration would also be beneficial in that it together with the jacket would result in a monolithic sheath that prevents separation of the component elements of the sheath by relative slippage therebetween during installation in the field.
As for the prior art, W. E. Petersen in U.S. Pat. No. 3,785,048, which issued on Jan. 15, 1974, overforms the outer longitudinal edge portion of a metallic tape, after which it is reverseformed by passage through an overlapping die in which a forming bar causes the edge portion to have a degree of permanent set. In K. P. Trusch patent 4,100,003 issued July 11, 1978, an outer longitudinal edge portion of a metallic tape along an overlapped seam is directed toward an inner edge portion a distance sufficient to preclude the outer edge portion from protruding into a subsequently extruded jacket. However, the inner longitudinal edge portion at the seam is unsupported during the seam forming and tends to be pushed in toward the core as the outer edge portion is directed inwardly.